https://www.irs.gov/newsroom/millions-of-people-will-get-their-economic-impact-payment-by-prepaid-debit-card

The determination of which taxpayers receive a debit card was made by the Bureau of the Fiscal Service, another part of the Treasury Department that works with the IRS to handle distribution of the payments.

No explanation of the reasoning. Apparently done, so we don't get checks signed by POTUS